The Caliciviridae is a family of positive-strand RNA viruses and consists of five genera designated: (1) Norovirus (with species Norwalk virus); (2) Sapovirus (with species Sapporo virus); (3) Vesivirus (with species, feline calicivirus and vesicular exanthema of swine virus); (4) Lagovirus (with species rabbit hemorrhagic disease virus and European brown hare syndrome virus) and (5) Nebovirus (with species Newbury-2 virus). The diseases caused by caliciviruses vary, according to the virus and its host species. Members of the Caliciviridae causing diarrheal disease in humans belong to the genera Norovirus and Sapovirus. The Caliciviruses Section (CS) in LID conducts research related to caliciviruses, with an emphasis on the noroviruses because of their predominant role in human diarrheal disease. Noroviruses are genetically-diverse and most human norovirus pathogens belong to either Genogroup I (GI) or Genogroup II (GII), with GII.4 as the predominant genotype. An important area of investigation in our laboratory is the role of genetic diversity in the natural history of norovirus infection, as this information is directly relevant to vaccine design. To address this question, we applied a genomics approach to study genetic and antigenic variation among noroviruses in different populations: the young, the elderly, and the immunocompromised. Furthermore, we examined noroviruses in various settings: hospitals, institutions, nursing homes and the community. We used next generation sequencing to conduct a high resolution analysis of diversity at each position of the approximately 7.6 kb RNA genome. We have identified regions of high variability during transmission events and in chronic infection that may give insight into host pressures that influence the emergence of epidemic genotypes. It should be noted that the extensive archival collection of noroviruses maintained by LID have provided some of the oldest known norovirus strains that inform evolutionary clock analyses and that established the ancestral origin of a number of current circulating recombinant strains. This year we made progress in setting up a pipeline for high throughput siRNA screening and transcriptome analysis of murine norovirus (MNV), a replication model for the caliciviruses because it grows efficiently in cell culture. The screening system established for MNV will be applied to studies of human noroviruses as permissive cell culture systems are developed and optimized. Our laboratory gained expertise in stem-cell derived intestinal organoids and the replication of human noroviruses in these cells is under investigation. We and our collaborators here at NIH were unable to verify that the cell line BJAB (derived from human B cells) is permissive for human norovirus replication, in spite of exhaustive efforts to replicate the system with the precise reagents from the laboratory where it was developed and reported. We completed an epidemiological investigation of noroviruses in immunocompromised patients enrolled in research protocols at the NIH Clinical Center. This work was prompted by concern that noscomial transmission might occur in the hospital in spite of stringent infection control measures. Our analysis showed that the overall prevalence rate (determined by whether the clinician ordered a diagnostic test for norovirus) was approximately 12% in this patient cohort. However, each patient shed a unique strain and many of the infections were chronic. Our investigation suggests that the majority of the chronic norovirus infections were community-acquired and not hospital-acquired. We are expanding our studies of these patients to understand the pathogenesis of norovirus and how the virus adapts to become chronic. We aim to identify an effective therapeutic option for these patients.